Laminated safety glass, such as used in vehicle windshields, is typically two mating sheets of glass bonded together with an interlayer of a thermoplastic material, such as polyvinyl butyral resin (PVB). When used as interlayer for safety glass, polyvinyl butyral resin is typically highly plasticized to make it soft and tacky. Laminated safety glass helps to protect from possible injury or other undesirable results of shattered glass resulting from forceful impacts. If safety glass is broken by impact, the interlayer remains substantially bonded to the broken glass, providing a form of protection from sharp and potentially dangerous shards. If in an accident an occupant is thrown into the windshield, the glass should yield somewhat to minimize impact injuries. Optimization between these two requirements requires careful control of the resin-to-glass adhesion. Adhesion that is too low may not provide an effective line of protections from shard. If adhesion is too high the unyielding windshield may worsen impact injuries. To achieve this control of adhesion, adhesion-limiting salts are typically added during the sheet forming extrusion step. Typical salts are alkali and alkaline earth salts, such as potassium or magnesium acetate, added in an amount sufficient to provide a cation concentration of about 100-500 mg/kg resin.
Plasticization of PVB is considered essential to its use in most types laminated safety glasses in which it is used. Plasticization of PVB can be accomplished by any number of known methods. For example, it is well known in the art to blend the plasticizer and the resin in a mixer, such as a Henschel or Banbury mixer, with subsequent extrusion compounding as described in U.S. Pat. No. 2,453,569. A solvent process, described by Haux in U.S. Pat. No. 2,046,378, used a C1 to C3 alcohol to introduce the plasticizer. Direct extrusion compounding is described in U.S. Pat. No. 5,886,075A. Another procedure is by absorption of the plasticizer into the resin from an aqueous slurry followed by a de-watering step and extrusion compounding.
In an aqueous slurry process, it has been found that different plasticizers are absorbed into the resin at different rates, thereby creating capacity and cost penalties. An aqueous slurry procedure has been described by Marks in U.S. Pat. No. 2,864,784, and describes the use of a stirred aqueous slurry of resin and an essentially water-insoluble plasticizer.
In the slurry process, the absorption rate of the plasticizer from the slurry into the resin is controlled by a number of parameters including ratio of solids (acetal resin and plasticizer) to water, plasticizing temperature, time, agitation speed and the like. The practical temperature range for aqueous plasticization as practiced in the prior art is from about 20° to 65° C. Due to the practical limitations of the conventional aqueous plasticization process, once a plasticizing system has been defined the only ways to increase capacity have been to add additional plasticizing vessels or increase agitation. Both of these options are costly and, in some instances, impractical due to structural limitations. The aqueous slurry process is also subject to significant variability in rate at which different plasticizers migrate into the resin.
After absorption of the plasticizer is complete, the plasticized resin is in the form of a plasticized slurry containing about 10% to about 25% solids (wet plasticized resin) and about 90% to about 75% of the aqueous phase. This slurry is first fed to a de-watering system that separates and removes 85-90% of the aqueous phase. The remaining aqueous phase, about 10% to about 15%, is entrained in the slurry solids that are forced into an extruder feed throat. This entrained water is removed by vacuum extraction in the extruder process. The extrudate is formed into sheeting by use of a slot sheeting die.
It is desirable to improve the process for the plasticization of polyvinyl butyral resins and to introduce adhesion-limiting salts in the process, thereby eliminating the subsequent separate step in which the adhesion-limiting salts must be added and thoroughly mixed. The present invention provides such a process.